In response to the increasing demand for corporate data networking, network providers are offering Frame Relay service as a more economical alternative to private lines for metropolitan and wide area networking. Frame Relay service is currently defined with access speeds ranging from 56 Kb/s to 2 Mb/s, although the technology itself is can operate at higher speeds. In addition, Frame Relay service offers multiple Committed Information Rates (CIRs) and Excess Information Rates (EIRs) for Frame Relay connections. CIR is the rate (expressed in bits/sec) at which the network agrees to transfer information under normal conditions. The rate is measured over the measurement interval T. CIR is negotiated at call establishment or service subscription time. The high priority traffic of a connection can be guaranteed a throughput equal to the Committed Information Rates (CIR). Excess Information Rates (EIR) is defined to allow for some flexibility to support bursty traffic such as data. EIR is the rate at which the network will attempt to deliver. The Frame Relay network will ensure that low priority traffic will be discarded, if necessary, before high priority traffic. Low priority frames are identified by the Discard Eligibility (DE) flag set to 1, whereas high priority frames have the DE flag set to 0. High priority traffic in excess of the CIR will be carried as low priority traffic through the Frame Relay network and thus excess information frames may be marked discard eligible (DE) by the network. Frame Relay users may also generate low priority traffic.
As in other network services, the Frame Relay service requires congestion management to maintain quality-of-service (e.g., throughput, delay, probability of frame loss). In order to ensure that Frame Relay users adhere to the subscribed service bit rates (CIR and EIR) and that service subscription violation by some users will not cause performance or throughput degradation on other user's services, the Frame Relay network will have to implement a rate (bandwidth) enforcement function at the access into the network. This rate enforcement may be achieved by discarding frames (with preference to those marked as DE) during periods of congestion.